This invention relates to the fixation of implants in bone. It particularly relates to fixation of the femoral component of a total replacement hip joint.
As total hip replacement becomes a procedure that is used in patients who are younger when the operation is performed, so it becomes necessary to have a method of fixation for the components of the implant in bone which is reliable and safe over very extended periods, e.g. for 20, 25 or 30 years or more. Conventional total replacement hip joint femoral stems are most often fixed in position inside the femur with polymethylmethacrylate bone cement. It has been suggested by a number of investigators that the use of bone cement may not give adequate fixation for the very long periods now being contemplated, and that a fixation system which does not use bone cement would be preferable. A number of such cementless fixation systems have been developed by various inventors throughout the world using either a "press fit" system or an "ingrowth" system. The press fit system relies on the surgeon producing an accurately shaped cavity in the femur into which the stem of the femoral implant will fit exactly. Loads are then transmitted from the implant stem directly to the bone. The ingrowth system relies upon a somewhat less accurate fit, but still a close fit, of the implant in the bony cavity. Bone is then expected to grow up to the implant surface, which is usually rough or structured, and lock the implant in place; or in a variation of this system, bone grows up to the implant surface and penetrates into the surface which is made porous to allow this penetration or ingrowth to take place. There are a number of difficulties with both the press fit and the ingrowth systems. Both need a large number of variations of size of femoral component to cope with the large variation of size of femur found in the group of patients to be treated. If a sufficient range of implants is not available then proper support of the implant by good quality, strong bone cannot be achieved in every case. If bone is to grow into, or onto, the surface of the implant, then the relative movement between the implant and the bone must be very small. In particular, shearing movement must be avoided. This means that a patient with a conventional "cementless" stem must be prevented from weight bearing for extended periods which is commonly not possible, and is usually contra-indicated in elderly patients. Because the shape of the cavity which has to be produced in the bone of the femur is essentially an irregular non-uniform but generally rectangular cross-section at the proximal (i.e. upper) end and essentially a circular, reasonably uniform cross-section at the distal (i.e. lower) end it is easy to obtain close fitting of the implant at the distal, circular end, but considerably more difficult at the proximal, irregular shaped end. However, it is necessary to transmit load at the proximal rather than the distal end to ensure disuse osteoporosis, or disuse atrophy, does not take place. Since load transmission with conventional cementless devices relies on the closeness of the fit of the implant in the bone, it is common to obtain better load transmission at the wrong end of the implant with potentially serious consequences to the patient. If bone is encouraged to grow up to and into the whole surface of an implant, the total surface area of metal in direct contact with the bone is very large. Consequently, the surface area available for corrosion, or which can take part in ionic transfer processes, is very large and the possibility of sensitisation of the patient to the materials of the implant is increased. Finally, the fatigue strength of the implant may be reduced by the effect of the rough or porous or sintered surface which is given to the implant.